The reverse DNA gyrases of thermophilic bacteria are enzymes that introduce positive supercoiling into circular DNA at the expense of ATP hydrolysis. We previously described an enzyme of this class with an unusual two-subunit structure in the hyperthermophile Methanopyrus Kandleri. The larger subunit (MW 138,000) has the ATPase activity while the smaller (MW 41,000) carries out the DNA breakage-reunion step. This functional partition is similar to that of the eubacterial DNA gyrases. We have now cloned and sequenced the genes for both subunits. Remarkably, the characteristic domain structure found in a single protein chain in all other type I topoisomerases is divided between the two subunits in this enzyme. Another topoisomerase from the same bacterium, topoisomerase V, has been shown to retain activity at temperatures as high as 122 degree C.